- #1

Oddbio

Gold Member

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**For any set**

*I*, and any family [itex](A_{i})_{i\in I}[/itex] of open subsets, the union [itex]\cup_{i\in I}A_{i}[/itex] is also an open set*(any union of open sets is open);*I also see one later like:

[tex]\cap^{n}_{i=1}A_{i}[/tex]

But I always thought that a union (and intersection) had to be of the form:

[tex]A\cup B or A\cap B[/tex]

So the first one has nothing on the left side, same as the second one.. but the second one also has super and sub-scripts?

Do they mean something completely different than "union" and "intersection"?

I apologize if this is a trivial question.. it sure feels like it.